1. Field of the Invention
The present invention relates to a free ink type writing instrument in which an ink stored in an ink tank is fed directly to a pen tip or a writing instrument provided with an ink tank of a cartridge type for storing an ink, wherein visibility of an amount of the ink is excellent.
2. Description of the Related Art
In general, it can be given as one of the characteristics of a free ink type writing instrument that an amount of the ink can be observed (visibility). That is, in a free ink type writing instrument, an ink is filled in an ink tank which is a holder part, and the ink freely moves in the ink tank; and when the pen tip is turned downward, the amount of the ink can visually be confirmed through a visible part of the holder part.
At present, polypropylene which is excellent in transparency and solvent resistance and which has a good moldability is used for a holder part for a free ink type writing instrument since the visibility can be secured.
In the present situation, however, only a free ink type writing instrument using a water-based ink having a fluorescent color or a bright color such as a pen for underlining makes it possible to visually confirm relatively easily an amount of the ink, while there involved is a problem that when used are a water-based ink having a dark color such as black and blue and an oil-based ink containing an organic solvent as a main solvent, a free ink type writing instrument does not have sufficiently high visibility of the amount of the ink yet and has not reached a practical use level.
It has so far been known to provide a rib on an internal surface of an ink tank part to elevate visibility in order to solve these problems, but the effect thereof has not yet been satisfactory.
Further, there have so far been tried various devices such as using a fluororesin for an ink tank material, treating with fluorine or silicone oils in order to provide an internal surface with a low surface energy state (Japanese Patent Publication No. Sho 38-20913) and using the rib described above in combination with silicone oil treatment (Japanese Patent Application Laid-Open No. Hei 11-216987).
However, in such devices as described in these publications, it takes time until an ink is repelled and slides down (clear draining) from an internal surface of an ink tank, and the effect on visibility thereof is unsatisfactory; a tank material does not have sufficient transparency, so that the effect thereof is not sufficiently persistent; and moldability of the material is inferior to require complicated steps, so that there involved is a problem in terms of a cost. Thus, the existing state is that satisfactory solutions have not yet been proposed.
In light of the problems and the status quo of the conventional techniques described above, the present invention intends to solve the problems, and an object thereof is to provide a writing instrument which is excellent in visibility of an ink and has excellent persistency of the effect thereof.
Intensive researches conducted by the present inventors in order to solve the problems on the conventional techniques described above have resulted in finding that a writing instrument which meets the object described above can be obtained by specifying a critical surface tension of the whole or a part of an internal surface part in contact with an ink in an ink tank part and a contact angle between the ink and the whole or a part of the internal surface part in the ink tank part. Thus, the present invention has been completed.
That is, the present invention comprises the following items (1) to (5). (1) A free ink type writing instrument having an ink tank part for storing an ink, wherein the whole or a part of an internal surface part in contact with the ink in the ink tank part has a smaller critical surface tension than a surface tension of the ink, and a contact angle between the ink and the whole or a part of the internal surface part in contact with the ink in the ink tank part is 30xc2x0 or more at a temperature of 25xc2x0 C. (2) The writing instrument as described in the above item (1), wherein the whole of the member of the ink tank part or the whole or a part of the internal surface part of the ink tank part comprises polymethylpentene. (3) The writing instrument as described in the above item (1) or (2), wherein the ink has a surface tension of 50 mN/m or less at a temperature of 25xc2x0, and a contact angle between the ink and the internal surface part in contact with the ink in the ink tank part is 30xc2x0 to 90xc2x0 at a temperature of 25xc2x0 C. (4) The writing instrument as described in any of the above items (1) to (3), wherein the ink has a viscosity of 1000 mPa""s or less at a temperature of 25xc2x0. (5) The writing instrument as described in any of the above items (1) to (4), wherein a principal solvent of the ink comprises an organic solvent.
In the present invention, a xe2x80x9cfree ink type writing instrumentxe2x80x9d means a writing instrument which has an ink tank for directly storing an ink without absorbing in a sliver and in which installed is an ink reservoir (collector member) for temporarily reserving the ink pressed out from the ink tank when the air contained in the ink tank is expanded by a rise in the temperature so as to prevent the ink from blobbing from the pen tip or the air hole.
The xe2x80x9ccritical surface tensionxe2x80x9d specified in the present invention means a critical surface tension xcex3c represented by a value of xcex3 corresponding to cos xcex8=1 on a straight line obtained by measuring contact angles xcex8 of liquids on the surface of a solid which is hardly wet to water and plotting (Zisman plot) values of cos xcex8 against surface tensions xcex3 of the liquids.
According to the present invention, provided is a writing instrument which is excellent in visibility of an ink and persistency of the effect thereof and which is prevented from being subjected to surface treatment of the whole or a part of the ink tank and therefore can be produced at a low cost without complicated after-steps.